Find and present some real examples where the difference in allele frequency between natural populations (any species) has been attributed to genetic drift. Explain why the scientists do not think the difference is due to selection.
In general, genetic drift refers to a change in allele frequency in a population over time due to random sampling. It can cause an increase, a decrease or even a loss in particular alleles, resulting in different allele frequencies in a population.
As a short term result, genetic drift would potentially cause an increase or a decrease, in a particular allele, therefore causing a change in allele frequency. In a naturally occurring population Allele frequency is the amount of copies of one gene that share a particular form. This occurs randomly through reproduction in any natural population. However, a long term effect would be one of less genetic variation in the population. This is where the bottle neck effect comes into play. As reproduction occurs, alleles will be passed on to each generation, in which particular alleles will not be passed on, making that allele rare in the population by chance.
Genetic Drift can cause these gene variants to disappear and therefore this will decrease the genetic variation, whereas natural selection makes gene variants more common or less common depending on success of reproduction and also the environment it's in. In small populations, genetic drift is larger because these random events that occur are representing a larger portion of the total population and also this could mean that alleles could be lost completely.
The diagram to the right shows the Population Bottleneck Effect. In this populations are subjected to a reduced population for at least one generation and even if the bottleneck doesn't last for many generations the population has already had a reduction in the genetic variation. Looking at the first bottle which says 'Parent Population' you can see that it is filled with yellow and blue balls but the majority of balls are yellow, now as we go to the second stage we can see that only a handful of balls are being randomly selected so now there has been a huge reduction in the population. Taking a look at the next stage which is the 'Surviving Individuals' we can see that there are 9 balls remaining and only 2 are yellow, so that allele could have potentially been wiped out but in this case it hasn't. In the last stage of this diagram, 'Next Generation' we can see that the bottle is filled again with balls and two thirds of the jar is now blue so that will be the dominating allele. So looking at this simple four step process we can see that there is a huge change in allele frequency and it's something that occurs very randomly and it will decrease the genetic variation.
One example of a species that has undergone the bottleneck effect in 1890 is the Northern Elephant Seals. They have been subjected to a reduction in genetic variation due to human infliction. Nearing the end of the 19th century these Northern Elephant Seals had a population size of around only 20 and this reduction in population was all due to human hunting. Their population has now bumped up again to 30,000 but looking at their genes, the bottleneck they went through is still visible as there is a substantial reduction in genetic variation compared to the Southern Elephant Seals which weren't intensely hunted.Yes but what is the evidence for the genetic drift.. what has happened to the allele frequencies? (most loci are fixed for one allele)
Trying to identify if a population is going through genetic drift is quite tough because it is such a slow procedure. Once the population that has survived the bottleneck reproduces the allele frequency compared to the parent population will be completely different. unclear, why does this make it hard to identify such species ? Another example of a species that has undergone a population bottleneck is the Cheetah. As this population had undergone a bottleneck, the species had lost a lot of genetic variation what is the evidence of that ?? (allozyme patterns)... If so, how do you interpret the higher microsatelllite diversity? so therefore if one of the cheetahs' was to undergo a skin graft, when it's returned back to the pack, the other cheetahs wouldn't recognise that it's foreign and therefore there would be no type of rejection for this cheetah. So in this example it has gone through a population bottleneck so drastically that cheetahs' wouldn't even be able to tell if it's a foreign cheetah that isn't part of their pack.
As stated before it is extremely difficult to find some sort of evidence that shows that genetic drift is occurring naturally in populations. Genetic drift is being used to explain the blood groups that have been recognised in Icelandic Cattle in comparison to their ancestors from which they were thought to have derived which were the Original Norwegian Cattle 1000 years ago. Upon examination the allele frequency at 8 blood group loci were examined. No known positive effects to the cattle have been noted due to the different alleles upon examination. Between the two populations today the allele frequency does not make sense. The allele frequency difference? is quite small and is appropriate to the amount of genetic divergence that occurred from genetic drift.
One example of a differences in allele frequency is present is in the African elephant (Elephas maximus) species. Most male elephants have tusks, whereas in most females they were absent, causing two different phenotypes; tusked and tuskless. It was ruled out that it was the sex of the animals that determined the presence of the tusks, as the few tusked females gave birth to tuskless offspring. logic unclear here Although genetic drift may not be the only cause of the different allele frequency, scientists believed it was one theory that explained it. One reason is because the founding population was very small, comprising of a population of 11 and resulting in the population going through a significant ‘bottle neck’ event allowing genetic drift to be more significant. how does this relate to females having tusks. You have not made your case.
Another quite different type of species this time is a type of vegetation, which is the Asian Bramble, this species illustrates genetic drift. It was a foreign weed that was introduced to some Pacific Islands. This time in order to study the Genetic Variation they referred to a method of DNA fingerprinting known as 'Amplified Fragment Length Polymorphisms' (AFLP). Polymorphisms will be identified by the appearance of a band or not. Vertical lines on the gel show an individual’s DNA and each AFLP shows a DNA fragment. The diagram on the right shows the native species which shows high polymorphism and the weed that was introduced (on the left) shows no polymorphism occurring, so this shows that the founder population was small and variation has been lost from it. This is a nice example, much better explained. Another similar phenomenon is the Founder Effect which is a type of Genetic Drift. It is where a new very small population is created from a much larger one, but this new population is now separate from the larger population and therefore genetic variation is dramatically reduced. As this new population is quite small it is more exposed to Genetic Drift. In this diagram on the right which is illustrating how a founder population is produced. We can see that the large circle contains the main original population and the three small circles have pinched off to create three very small founder populations. In two of the circles we can see that there is only one allele type whereas the other circle contains a mixture of two alleles, this here shows us straight away that this effect will, like genetic drift, reduce the amount of genetic variation in the new populations.
An example of this founder effect is the Hawaiian Drosophila. A female Drosophila fly is able to carry the sperm that it has collected from the previous multiple mating for a long period of time. So by chance this single female fly has somehow been blown away to a new island where this fly hasn't yet been inhabited. so this single female fly is carrying the sperm so the genes from two or three males with her so when she does reproduce the genetic diversity that will arise will be extremely limited but this will be the founder population for the new population that will be growing on this island. yes but how do we know it has occurred??
Not only does this founder effect occur in flora or fauna, but it also occurs in humans. In the Dominican Republic there is a village called the Salinas. It had an extremely small population about 40 years ago of just over 4000 people. Quite a few generations before this the village was even smaller. One of the main important founders during this time was a man called Altagracia Carrasco who fathered many children with four different women. As the population was so small his genes made up a large majority of the total gene pool for their population so there wouldn't have been much genetic diversity. Carrasco carried the frequent alleles that were now present in their population but he also carried a unique allele that causes mutation by single base substitution. This allele greatly affected only persons of XY. This allele develops the testes but doesn’t allow for too much of the DHT (masculinity) to be produced so the rest of the body develops in a female orientation. As this allele was in a high frequency in this population the XY persons were raised as girls as they looked female even though they did have internal testes. At around 12 years of age as the 'girls' started to reach puberty there was a high level of testosterone being produced by the testes and this helped the male sexual organs to develop further. So once they hit puberty they became boys. OK.. how do we know these are his children though?
Amish people are also a very small population group that have formed from a founder’s population. the founders population was to start with completely isolated from the original population, so therefore this group of people will have completely selected avoid the term 'selected' unless you mean natural selection has occurred. I think you mean a random sample here... so that is almost the opposite of the meaning you attempted to convey. alleles that are only a handful from the original population and their offspring will inherit only the limited amount of alleles that they carry so there isn't much chance at all of genetic variation. As these Amish people have descended from a small founder population they do have some very unusual alleles such as the Polydactyl, which is the allele for extra fingers. There is more evidence that drift has occurred here. E.g. allele frequencies of blood enzymes and conventional blood groups.
Reasons why some scientists would argue that it is genetic drift that has ultimately caused a difference in allele frequency is perhaps because in some circumstances, genetic drift can stop the process of natural selection occurring. no, it can counteract it. For example, in cases where genetic drift has occurred, alleles that may otherwise seem ‘unfavourable’ in the case of natural selection may increase in a population by chance. Therefore, some alleles remain in the population at high frequencies through reproduction than they would if selection had occurred.
Bibliography
<!--[if gte mso 10]> Whitehouse, A.M. (2001) Tusklessness in the elephant population of the Addo Elephant National Park, South Africa in Journal of Zoology 257: 249-254
Templeton, A.R (2006) Population genetics and micro evolutionary theory. John Wiley & Sons Inc. pp 82-92
Skelton, P (eds) (1993) Evolution: A biological and paleontological approach. 1st ed. New Jersey: Prentice Hall Inc
Find and present some real examples where the difference in allele frequency between natural populations (any species) has been attributed to genetic drift. Explain why the scientists do not think the difference is due to selection.
In general, genetic drift refers to a change in allele frequency in a population over time due to random sampling. It can cause an increase, a decrease or even a loss in particular alleles, resulting in different allele frequencies in a population.
As a short term result, genetic drift would potentially cause an increase or a decrease, in a particular allele, therefore causing a change in allele frequency. In a naturally occurring population Allele frequency is the amount of copies of one gene that share a particular form. This occurs randomly through reproduction in any natural population. However, a long term effect would be one of less genetic variation in the population. This is where the bottle neck effect comes into play. As reproduction occurs, alleles will be passed on to each generation, in which particular alleles will not be passed on, making that allele rare in the population by chance.
Genetic Drift can cause these gene variants to disappear and therefore this will decrease the genetic variation, whereas natural selection makes gene variants more common or less common depending on success of reproduction and also the environment it's in. In small populations, genetic drift is larger because these random events that occur are representing a larger portion of the total population and also this could mean that alleles could be lost completely.
The diagram to the right shows the Population Bottleneck Effect. In this populations are subjected to a reduced population for at least one generation and even if the bottleneck doesn't last for many generations the population has already had a reduction in the genetic variation. Looking at the first bottle which says 'Parent Population' you can see that it is filled with yellow and blue balls but the majority of balls are yellow, now as we go to the second stage we can see that only a handful of balls are being randomly selected so now there has been a huge reduction in the population. Taking a look at the next stage which is the 'Surviving Individuals' we can see that there are 9 balls remaining and only 2 are yellow, so that allele could have potentially been wiped out but in this case it hasn't. In the last stage of this diagram, 'Next Generation' we can see that the bottle is filled again with balls and two thirds of the jar is now blue so that will be the dominating allele. So looking at this simple four step process we can see that there is a huge change in allele frequency and it's something that occurs very randomly and it will decrease the genetic variation.
One example of a species that has undergone the bottleneck effect in 1890 is the Northern Elephant Seals. They have been subjected to a reduction in genetic variation due to human infliction. Nearing the end of the 19th century these Northern Elephant Seals had a population size of around only 20 and this reduction in population was all due to human hunting. Their population has now bumped up again to 30,000 but looking at their genes, the bottleneck they went through is still visible as there is a substantial reduction in genetic variation compared to the Southern Elephant Seals which weren't intensely hunted.Yes but what is the evidence for the genetic drift.. what has happened to the allele frequencies?
(most loci are fixed for one allele)
Trying to identify if a population is going through genetic drift is quite tough because it is such a slow procedure. Once the population that has survived the bottleneck reproduces the allele frequency compared to the parent population will be completely different. unclear, why does this make it hard to identify such species ? Another example of a species that has undergone a population bottleneck is the Cheetah. As
As stated before it is extremely difficult to find some sort of evidence that shows that genetic drift is occurring naturally in populations. Genetic drift is being used to explain the blood groups that have been recognised in Icelandic Cattle in comparison to their ancestors from which they were thought to have derived which were the Original Norwegian Cattle 1000 years ago. Upon examination the allele frequency at 8 blood group loci were examined. No known positive effects to the cattle have been noted due to the different alleles upon examination. Between the two populations today the allele frequency does not make sense. The allele frequency difference? is quite small and is appropriate to the amount of genetic divergence that occurred from genetic drift.
One example of a differences in allele frequency is present is in the African elephant (Elephas maximus) species. Most male elephants have tusks, whereas in most females they were absent, causing two different phenotypes; tusked and tuskless. It was ruled out that it was the sex of the animals that determined the presence of the tusks, as the few tusked females gave birth to tuskless offspring. logic unclear here Although genetic drift may not be the only cause of the different allele frequency, scientists believed it was one theory that explained it. One reason is because the founding population was very small, comprising of a population of 11 and resulting in the population going through a significant ‘bottle neck’ event allowing genetic drift to be more significant. how does this relate to females having tusks. You have not made your case.
Another quite different type of species this time is a type of vegetation, which is the Asian Bramble, this species illustrates genetic drift. It was a foreign weed that was introduced to some Pacific Islands. This time in order to study the Genetic Variation they referred to a method of DNA fingerprinting known as 'Amplified Fragment Length Polymorphisms' (AFLP). Polymorphisms will be identified by the appearance of a band or not. Vertical lines on the gel show an individual’s DNA and each AFLP shows a DNA fragment. The diagram on the right shows the native species which shows high polymorphism and the weed that was introduced (on the left) shows no polymorphism occurring, so this shows that the founder population was small and variation has been lost from it. This is a nice example, much better explained.
Another similar phenomenon is the Founder Effect which is a type of Genetic Drift. It is where a new very small population is created from a much larger one, but this new population is now separate from the larger population and therefore genetic variation is dramatically reduced. As this new population is quite small it is more exposed to Genetic Drift. In this diagram on the right which is illustrating how a founder population is produced. We can see that the large circle contains the main original population and the three small circles have pinched off to create three very small founder populations. In two of the circles we can see that there is only one allele type whereas the other circle contains a mixture of two alleles, this here shows us straight away that this effect will, like genetic drift, reduce the amount of genetic variation in the new populations.
An example of this founder effect is the Hawaiian Drosophila. A female Drosophila fly is able to carry the sperm that it has collected from the previous multiple mating for a long period of time. So by chance this single female fly has somehow been blown away to a new island where this fly hasn't yet been inhabited. so this single female fly is carrying the sperm so the genes from two or three males with her so when she does reproduce the genetic diversity that will arise will be extremely limited but this will be the founder population for the new population that will be growing on this island. yes but how do we know it has occurred??
Not only does this founder effect occur in flora or fauna, but it also occurs in humans. In the Dominican Republic there is a village called the Salinas. It had an extremely small population about 40 years ago of just over 4000 people. Quite a few generations before this the village was even smaller. One of the main important founders during this time was a man called Altagracia Carrasco who fathered many children with four different women. As the population was so small his genes made up a large majority of the total gene pool for their population so there wouldn't have been much genetic diversity. Carrasco carried the frequent alleles that were now present in their population but he also carried a unique allele that causes mutation by single base substitution. This allele greatly affected only persons of XY. This allele develops the testes but doesn’t allow for too much of the DHT (masculinity) to be produced so the rest of the body develops in a female orientation. As this allele was in a high frequency in this population the XY persons were raised as girls as they looked female even though they did have internal testes. At around 12 years of age as the 'girls' started to reach puberty there was a high level of testosterone being produced by the testes and this helped the male sexual organs to develop further. So once they hit puberty they became boys. OK.. how do we know these are his children though?
Amish people are also a very small population group that have formed from a founder’s population. the founders population was to start with completely isolated from the original population, so therefore this group of people will have completely selected avoid the term 'selected' unless you mean natural selection has occurred. I think you mean a random sample here... so that is almost the opposite of the meaning you attempted to convey. alleles that are only a handful from the original population and their offspring will inherit only the limited amount of alleles that they carry so there isn't much chance at all of genetic variation. As these Amish people have descended from a small founder population they do have some very unusual alleles such as the Polydactyl, which is the allele for extra fingers. There is more evidence that drift has occurred here. E.g. allele frequencies of blood enzymes and conventional blood groups.
Reasons why some scientists would argue that it is genetic drift that has ultimately caused a difference in allele frequency is perhaps because in some circumstances, genetic drift can stop the process of natural selection occurring. no, it can counteract it. For example, in cases where genetic drift has occurred, alleles that may otherwise seem ‘unfavourable’ in the case of natural selection may increase in a population by chance. Therefore, some alleles remain in the population at high frequencies through reproduction than they would if selection had occurred.
Bibliography
<!--[if gte mso 10]>
Whitehouse, A.M. (2001) Tusklessness in the elephant population of the Addo Elephant National Park, South Africa in Journal of Zoology 257: 249-254
Templeton, A.R (2006) Population genetics and micro evolutionary theory. John Wiley & Sons Inc. pp 82-92
Skelton, P (eds) (1993) Evolution: A biological and paleontological approach. 1st ed. New Jersey: Prentice Hall Inc
Caldwell, R et al., Bottleneck and Founder Effects [Online] 2006. Available from: http://evolution.berkeley.edu/evosite/evo101/IIID3Bottlenecks.shtml. [Accessed: 10th January 2011]
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